Flat panel image sensors are commercially successful products that are able to effectively detect electromagnetic radiation in and near the visible light spectrum. Flat panel image sensors are fabricated by depositing and patterning various metals, insulators, and semiconductors on glass substrates as is done in flat panel displays. Such sensors commonly employ photosensitive elements, such as amorphous silicon (a-Si) PIN diodes. The photosensitive element is coupled to a readout switch, such as thin film transistor (TFT), that provides data indicative of received light.
A common use for flat panel image sensors is for medical and industrial applications to detect x-rays. The image sensor includes a phosphorescent screen that overlays an array of image sensing elements. The phosphorescent screen converts received x-rays to visible light. The array receives the visible light and generates a photocurrent responsive to the light. The photocurrent is read out as data indicative of the sensed light.
The arrays are difficult to manufacture since separate process steps are required to construct the PIN diodes and the TFTs. The total mask count may be 8 or more which is burdensome while the yields are low. Furthermore, a-Si PIN diodes are not a standard device in flat panel display processing which increases manufacturing expense. It would therefore be advantageous to use a standard process to greatly reduce the cost of flat panel image sensors.
Manufacturing TFTs for flat panel display applications is a common process. A common use for TFTs is in active matrix liquid crystal displays (AMLCDs). Each TFT functions as a switch for a pixel in a matrix display. The voltage across each pixel is controlled independently and at a high contrast ratio. TFTs may be fabricated by depositing and patterning metals, insulators, and semiconductors on substrates through methods well known in the art. TFTs typically employ a-Si, polycrystalline silicon, or CdSe film as the semiconductor material. A-Si is typically used in flat panel display applications as it is easily deposited on large area glass substrates at temperatures below 350 centigrade.
TFTs are more economical to fabricate than a-Si PIN diodes and are well suited for flat panel applications. The present inventors have recognized that if both the image sensing element and the readout switch of an image sensor array were incorporated as TFTs, fewer photomasks would be required and manufacturing costs would be greatly reduced.
TFTs have not typically been used as photosensitive elements. U.S. Patent Application Publication Nos. 2001/0055008 and 2001/0052597, both to Young et al. (hereinafter the “Young applications”) disclose the use of TFTs as light sensing elements for an emissive display device. The light sensing elements provide feedback to progressively adjust the current flow through display elements to control light output. However, the use of TFTs exclusively for an image sensor is not disclosed. Since a TFT is more economical to manufacture and has already been successfully incorporated into flat panel applications, the present inventors have recognized that it would be advantageous to employ TFTs in image sensors.
Other applications disclose detecting light reflected through a transparent sensor element incorporating a TFT. J. H. Kim et al., “Fingerprint Scanner Using a-Si:H TFT array,” SID '00 Digest, Long Beach, Calif., USA, pp. 353-355 discloses a contact image sensor that requires a transparent sensor element to pass reflected light. M. Yamaguchi et al., “Two-Dimensional Contact-Type Image Sensor Using Amorphous Silicon Photo-Transistor,” Jpn. J. Appl. Phys., Vol. 32 (1993) pp. 458-461 discloses an image sensor that passes reflected light through a transparent sensor element and also receives direct light.
Conventional image sensing applications have not considered the use of TFTs to detect relatively weak x-ray emissions. In order to detect x-ray emissions, the sensitivity of the imaging sensing TFT is a primary concern. Conventional devices are unable to provide adequate light detection for x-ray applications. Transparent pixels, in particular, do not provide sufficient sensor element density to detect light resulting from x-ray emissions. Thus, it would be an advancement in the art to provide a TFT image sensor with enhanced light detection capability and suitable for x-ray applications. Such a device is disclosed and claimed herein.